Underwater motor pumps having an output range of up to 10 kW are now mass produced for use in the water supply field, for example and the manufacturers of such pumps have accordingly sought to lower their production costs.
A first expedient to this end is to simplify the structure of a conventional pump, by ensheathing the motor stator by casting it in plastics material according to the teaching, for example, of U.S. Pat. No. 4,546,130.
A second expedient consists in providing a pump assembly with an integrated frequency changer to enable the pump to be operated at a higher rotational speed in order to increase the hydraulic power of the pump assembly. A pump assembly of this kind is disclosed in DE-A No. 36 42 727.
The second expedient is to be preferred, however, as will now be explained.
The following known relationships apply: EQU Q.about.n.D.sup.3 EQU H.about.n.sup.2.D.sup.2 EQU P.about.n.sup.3 D.sup.5
where Q is the pump delivery flow, H the delivery head, P the electrical power, n the rotational speed of the pump assembly and D the diameter of the pump impeller.
Identical output may be obtained whilst reducing the diameter of the pump assembly and/or reducing the number of stages of the pump, by increasing the rotational speed thereof. Installation costs may thereby be lowered since the cost of a well for drinking water supply, for example, is a substantial function of the well diameter.
Such cost cutting expedients give rise, however, to problems of cooling the pump assembly. The dispersal of waste heat generated in the motor of the assembly is inhibited by the thermal insulating action of the plastics material sealing means thereof by the reduction in the heat-emitting area of the assembly and by electrical heat losses in the frequency changer which is integrated with the motor of the assembly.